The invention also relates to a coating composition as obtained with said process, and to a process of applying a porous anti-reflective coating (ARC) on a substrate using such composition, and to the resulting coated substrate.
A typical example of an ARC is a thin layer of porous inorganic oxide —for example a layer of less than 0.2 μm thickness—which substantially consists of an inorganic oxide like silica and has certain porosity. Such coatings may be applied to a transparent substrate to reduce the amount of light being reflected from its surface, i.e. from the air-substrate interface, and thus increase the amount of light being transmitted through the substrate. Such coatings can be used as single layer or as part of a multi-layer coating (or coating stack). Typical single layer ARCs based on thin porous silica layers have been described in e.g. EP0597490, U.S. Pat. Nos. 4,830,879, 5,858,462, EP1181256, WO2007/093339, WO2008/028640, EP1674891, WO2009/030703, WO2009/140482, US2009/0220774, and WO2008/143429.
A single layer ARC on a transparent substrate typically should have a refractive index between the refractive indices of the substrate and air, in order to effectively reduce the amount of light reflected. For example, in case of a glass with refractive index 1.5 the AR layer typically should have a refractive index of about 1.2-1.3, and ideally of about 1.22. A porous silica (or other inorganic oxide) layer having sufficiently high porosity can provide such a low refractive index and function as AR coating, if its layer thickness is about ¼ of the wavelength of the light; meaning that in the relevant wavelength range of 300-800 nm the thickness preferably is in the range 70-200 nm.
Optimum porosity and pore size in an ARC is not only depending on the coating layer thickness, but also on other desired performance characteristics. For example, pore size should not be too large, to minimise light scattering and optimise transparency. On the other hand, if the layer contains very small pores, this may result—under ambient use conditions—in non-reversible moisture up-take via capillary condensation; affecting refractive index and making the coating layer more prone to fouling. Such capillary condensation effects have been reported for so-called meso-porous silica, especially having pores in the range 1-20 nm. Porosity is needed to reduce refractive index, but too high porosity level may deteriorate mechanical strength of the coating, e.g. reduce (pencil) hardness and abrasion resistance.
By inorganic oxide precursor is herein meant a compound that contains metal and can be converted into a metal oxide for example by hydrolysis and condensation reaction.
Porous AR coatings can be made from a solvent borne coating composition comprising an inorganic or organic binder material and a pore forming agent. In case of an inorganic binder, for example based on an inorganic oxide precursor, typically a sol-gel process is used for making a (porous) inorganic oxide layer, wherein a precursor compound in solution or colloid (or sol) form is formed into an integrated network (or gel) of either discrete particles or network polymers. In such process, the sol gradually evolves to a gel-like diphasic system containing both a liquid and solid phase. Removing remaining liquid (drying) is generally accompanied by shrinkage and densification, and affects final microstructure and porosity. Afterwards, a thermal treatment at elevated temperature is often applied to enhance further condensation reactions (curing) and secure mechanical and structural stability. Typical inorganic oxide precursors used are metal alkoxides and metal salts, which can undergo various forms of hydrolysis and condensation reactions. Metal is understood to include silicon within the context of this description.
Such AR coating composition contains solvent and organic ligands from organo-metallic inorganic oxide precursors, which compounds as such will induce some porosity to the inorganic oxide layer; but typically with pores smaller than 10 nm. The further presence of a pore forming agent in the coating composition will help in generating suitable porosity level and pore sizes in the final AR layer to provide the desired refractive index. Suitable pore forming agents, also called porogens, known from prior art publications include organic compounds, like higher boiling solvents, surfactants, organic polymers, and inorganic particles having sub-micron particle size, including porous particles and organic-inorganic core-shell nano-particles.
Use of porous or hollow nano-particles in a binder or matrix material represents an elegant way to control porosity level and pore sizes in a porous ARC layer. Various different synthetic strategies for making hollow inorganic particles can be distinguished, as for example described in Adv. Mater. 2008, 20, 3987-4019. A typical approach applies a micro- or nano-sized organic structure in a solvent system as a template or scaffold for forming an inorganic oxide outer layer (also referred to as coating with inorganic oxide), resulting in hybrid organic-inorganic core-shell (or coated) nano-particles as intermediate product. Shell layers comprising silica are generally made with a sol-gel process based on the so-called Stöber method, wherein an alkoxy silane is hydrolysed and condensed in water/alcohol mixtures containing ammonia.
A coating composition comprising pre-fabricated hollow inorganic nano-particles and a matrix-forming binder based on silica precursors is for example described in EP1447433. A disadvantage of this approach is that porous or hollow nano-particles, which can be re-dispersed in a liquid composition, are difficult to make.
Many documents address using organic-inorganic core-shell nano-particles with an organic polymer core as an alternative to making an AR coating composition. After applying and drying such coating composition on a substrate, the organic core can be removed from the coating by various methods; for example by exposing the coating to a solvent for the polymer and extracting it from the coating. Alternatively organic polymer can be removed during thermally curing the coating, for example at temperatures above the decomposition temperature of the polymer (i.e. by pyrolysis or calcination). Suitable temperatures are from about 250 to 900° C. A combined treatment of dissolving and degrading/evaporating the polymer may also be applied.
WO2008/028640 represents an example of a publication that describes applying such hybrid core-shell nano-particles with a sacrificial organic polymer core. In this document cationic polymer micelles or cationically stabilised polymer latex particles are used as organic template for making hybrid core-shell nano-particles and AR coating compositions comprising said core-shell particles and inorganic oxide precursor as binder. Thermally curing an applied layer results in a porous coating with in situ formed hollow particles embedded in the binder, wherein pore size is dependent on the size of the polymer template used for making the core-shell particles. A disadvantage of using nano-particles with a polymeric core is that for effective removal exposure to high temperatures may be needed, which limits its use with thermally sensitive substrate like thermoplastics.
JP 2011111558 discloses a coating composition where particles are based on a polymer emulsion particle based on polymerized vinyl monomer with a specific functional group, hydrolysable silicone compound, emulsifier and water.
JP2008274261 discloses a coating composition for forming a hollow silicone particle film. Examples show that the hollow silicone particles are prepared by first making a core of polymer particles using anionic or non-ionic emulsifiers and thereafter silicone precursors are added to form the shell, whereafter the core is removed by extraction or heating. A potential solution to above indicated thermal restrictions would include core-shell particles that have a more volatile core, for example comprising an organic compound with a boiling point below 300° C. Numerous research groups have published on synthesis and properties of hybrid core-shell particles and inorganic hollow nano-particles; see for example reviews such as Adv. Mater. 2008, 20, 3987-4019. Preparation of stable dispersions of suitable core-shell nano-particles based on low molar mass organic templates and having particle size well below about 500 nm appears however rather difficult.
Therefore, there remains a need in industry for an anti-reflective coating composition based on hybrid organic-inorganic core-shell nano-particles, which composition can be made into an ARC at relatively low temperatures, for example on thermally sensitive substrates.
It is thus an objective of the present invention to provide such an improved AR coating composition and a process of making it.
The solution to above problem is achieved by providing the process as described herein below and as characterized in the claims.
Accordingly, the present invention provides a process of making an anti-reflective coating composition comprising the steps of    1) Preparing an oil-in-water emulsion by mixing
an apolar organic compound A;
a cationic addition copolymer C as emulsion stabilizer; and
aqueous medium of pH 2-6;
at a mass ratio C/A of 0.1 to 2, to result in 1-50 mass % (based on emulsion) of emulsified droplets of particle size 30-300 nm; and    2) Providing an inorganic oxide shell layer to the emulsified droplets by adding to the emulsion obtained in step 1) at least one inorganic oxide precursor, to result in organic/inorganic core-shell nano-particles with mass ratio core/shell of from 0.2 to 25.
With the process of the invention it is found possible to make a coating composition comprising core-shell particles based on emulsified droplets that contain a liquid apolar organic compound and having an particle size in the range of 30 to 300 nm. This particle size can be controlled by the type of organic compound and the monomer composition of the cationic copolymer, and/or by selecting conditions like temperature, pH, aqueous medium composition, and to a lesser degree stirring rate. A further advantage of the present process is that the dispersion of core-shell nano-particles obtained is stable under different conditions; increasing its shelf life or storage time, and allowing for example altering its concentration and solvent system, and addition of different binders and auxiliary components. The coating composition obtained with the process according to the invention can be advantageously used for making porous ARCs at a wide range of temperatures on different substrates, including thermoplastic substrates that have low temperature resistance; because the organic compound can be easily removed from the core-shell particles by solvent extraction or evaporation at relatively low temperature, and low temperature curable binder can be used.